The invention concerns an apparatus and a method for inflating thermoformable material.
A known type of heating apparatus, such as shown in WO-A-94/08852, comprises heaters between which a pair of string at a pre-heating station of thermoformable material is inserted and indexed, the strips being heated to predetermined temperatures by contact with surfaces of heating blocks of the heaters. In the apparatus of this type, an intermediate heating block is provided, interposed between the pair of strips, the intermediate heating block receiving heat by radiation from the external heaters. Consequently, control of the temperature of the intermediate heating block is difficult and there is a tendency to overheat. After the pre-heating station is a heat-sealing station for heating and welding the pair of strips, comprising a pair of mould shells having respective recesses facing each other, thermally coupled with heating means and translated by driving means to close on the pair of strips and heat then while a pneumatic fluid is injected to between the strips by suitable injecting means. The injection of the pressurized fluid serves to prevent the hot central parts of the strips from contacting each other and undesirably welding together during, or after, heating. The pressure of the fluid which is injected between the strips is transmitted to the surfaces of the recesses and, in turn, the shells transmit the forces to their respective supporting plates through respective driving means in the form of cams and followers. Thus, the driving force counterbalancing the inflating pressure is concentrated over a relatively small area and therefore the areas of those portions of the strips which are to be heated cannot be excessive otherwise the fluid would exert extremely high forces on the shells which could damage the heat-sealing station. This tends to limit the productivity of the system. After the heat-sealing station is a pneumatic forming station to which the pneumatic fluid is supplied through grooves extending from the recesses in the shells of the heat-sealing station to recesses in a pair of shells of the forming station.
U.S. Pat. No. 3,829,264 discloses a blow-moulding apparatus for thermoplastic material and including a mould having two relatively movable mould sections. One or both of the mould sections are mounted on carriers for relative movement, and adjusting means in the form of springs or in the form of a bladder extensible by pressure fluid is interposed between at least one of the mould sections and its associated carrier so that the mould sections can be adjusted relative to each other and aligned in parallelism. The mould sections are mounted upon respective arms turnable about a common axis by driving means in the form of oppositely acting pressure-fluid motors.
In U.S. Pat. No. 3,245,197, the system includes a thermoforming station at which two strips have formed therein respective rows of cavities, followed by a welding and filling station at which the outlines of the pairs of cavities are welded together except at filling openings and the containers thus obtained are filled. The welding and filling station is followed by a closing station at which the filling openings are sealed. At the thermoforming station is a stationary mould member which is wedge-shaped and has its two opposed inclined mould faces facing towards respective mould faces of two turnable mould members. The stationary member has compressed air outlet apertures opening through its opposed inclined faces to supply jets of air to force the sheets into contact with the mould faces of the turnable members. Electrical heaters may be provided in the stationary member to keep the strips at the proper temperature for pressure moulding. The strips may be pre-heated at respective heating stations preceding the thermoforming station. Each turnable member is pivotally mounted on a rod. During indexing of the strips through the stations, the turnable members extend vertically downwards from the respective rods. At the end of each step of advance of the strips, the turnable members are swung into fluid-tight contact with the strips and clamp them against the stationary member. For swinging the movable members to-and-fro, they are pivotally connected to respective, horizontal, hydraulic piston-and-cylinder devices.
According to one aspect of the present invention, there is provided apparatus comprising first fluid supplying means for supplying inflating fluid to between opposite portions of thermoformable material, first and second parts for receiving said portions between them, one of the parts being in the form of a shell, said shell defining a recess having a surface against which said inflating fluid presses one of said opposite portions, driving means operable to translate the shell towards the other part, with said portions between the first and second parts, to bring the shell into a closed condition relative to the other part and to maintain the shell in said closed condition against the inflating pressure of the inflating fluid, and second fluid supplying means arranged to supply operating fluid to said driving means, characterized in that the driving means and the second fluid supplying means are pneumatic.
According to second aspect of the present invention, there is provided apparatus comprising first fluid supplying means for supplying inflating fluid to between opposite portions of thermoformable material, first and second parts for receiving said portions between them, one of the parts being in the form of a shell, said shell defining a recess having a surface against which said inflating fluid presses one of said opposite portions, driving means operable to translate the shell towards the other part, with said portions between the first and second parts, to bring the shell into a closed condition relative to the other part and to maintain the shell in said closed condition against the inflating pressure of the inflating fluid, said driving means including an operating chamber having an operating surface, and second fluid supplying means arranged to supply operating fluid to said operating chamber to translate said operating surface and thus said shell towards said other part, characterized in that the projected areas of said surface of said recess and of said operating surface are of substantially the same size as each other.
According to a third aspect of the present invention, there is provided a method comprising providing first and second parts one of which is in the form of a shell, supplying operating fluid to driving means to operate said driving means to translate the shell towards the other part, with opposite portions of thermoformable material being received between the first and second parts, to bring the shell into a closed condition relative to the other part, and supplying an inflating fluid to between said opposite portions to inflate said opposite portions while said driving means serves to maintain said shell in said closed condition, characterized in that the operating of said driving means to bring the shell into said closed condition and to maintain the shell in said closed condition is pneumatic.
The use of pneumatic fluid as compared with hydraulic fluid has the advantages that the pneumatic fluid is compressible and that, since in most cases in practice, the inflating fluid would be pneumatic fluid, the operating fluid can often be taken from the same source as the inflating fluid, thus considerably simplifying the apparatus. The arrangement whereby the projected area of the surface of the recess is of substantially the same size as the projected area of the operating surface has the advantage that the driving force can be spread over at least the projected area over which the inflating force is spread.
In a preferred embodiment of the invention, a heating apparatus for heating a pair of strips of thermoformable material comprises a pair of shells having recesses facing each other, the shells being thermally coupled with temperature varying means and driven by pneumatic driving means to close on the pair of strips and heat them while pneumatic inflating fluid is injected between the strips by injecting means in order to keep the strips mutually spaced apart and in contact with the respective internal surfaces of the pair of shells, the operating fluid acting in a direction opposite to the direction of the force exerted by the inflating fluid, the projected area acted on by the operating fluid being at least substantially equal to the projected area acted on by the inflating fluid, and a support for the driving means extending over an area at least substantially equal to the projected area acted on by the operating fluid. Thus, since the shell can bear against the driving means over a considerable area and since the driving means can bear against a fixed support over a considerable area, it is possible for those portions of the strips which are to be heated between the shells also to extend over a considerable area.
Owing to the invention, it is possible to increase considerably the productivity of a blow-thermoforming apparatus for forming containers, because an increase of the internal surfaces of the shellsxe2x80x94which allows the heating of strip portions having considerable areasxe2x80x94even if it involves an increase in the inflating force exerted on the shells, can be easily compensated for. Moreover, it is relatively simple to heat materials which are particularly difficult to heat, for example polypropylene, because a single heating station can extend for a plurality of indexing pitches.